DC/AC converters are commonly used for various applications such as renewable power conditioning systems, electric vehicles, etc. In particular, DC/AC inverters are widely used as the second stage in two-stage renewable energy power conditioning systems. The DC/AC inverter usually operates under hard-switching where neither the voltage nor the current of the power switches is zero during the switching transitions. The power semiconductors of the DC/AC inverter are switched under very high voltage at the intermediate DC-bus (usually more than 400V). Switching losses of the power semiconductors in such inverters significantly contribute to the overall losses of the power conditioning system. In particular, reverse recovery losses of the power semiconductors' body diodes are inevitable for such topologies. Because of this, the switching frequency of the inverter is very limited, usually in the range of 10-20 kHz. Because of strict regulatory standards, a high quality current needs to be injected into the utility grid from such inverters. To accomplish this, such inverters require large filters at their outputs. Another issue with low switching frequencies is that such low frequencies create a high amount of current ripple across the inverter output inductor. This current ripple not only increases the core losses of the inductor but it also increases the inductor's high frequency copper losses. In addition to this, reducing the DC-bus voltage creates a significant amount of conduction and emission EMI noise. The high amount of conduction and EMI noise may affect the operation of the control system and may highly degrade the system's reliability. From all of the above, hard-switching limits the switching frequency of the converter and imposes a substantial compromise in the design of the output filter and in the overall performance of the power conditioning systems.
There are many different soft-switching techniques reported in the literature. However, these techniques generally require many extra active/passive circuits. In particular, extra active circuits highly deteriorate the reliability of the system due to the additional complexity imposed by the active components. Also, the effectiveness of these techniques is questionable. Some studies have shown that some soft-switching techniques may add more losses to the converter and, consequently, greatly offset their advantages. Because of this, most industrial products use the more conventional hard-switching inverters with large filters in order to ensure a reliable power conditioning system. Even though the system's performance is highly compromised with hard-switching and bulky lossy filters, industrial decision makers prefer to use a reliable, well-known solution for the inverter.
Auxiliary circuits have previously been used to provide soft-switching conditions for the power semiconductors of a voltage source inverter. Such soft-switching circuits usually use a combination of active and passive circuits in order to provide soft-switching conditions. Generally, active circuits increase the complexity of the power circuit and reduce the reliability of the systems. In addition to this issue, losses related to the auxiliary circuits usually highly offset the advantages of the soft-switching and compromise the converter performance. Usually, auxiliary circuits include a resonant circuit with very high amount of peak current/voltage. From the above, it should be clear that significant amounts of losses are attributed to the auxiliary circuit. As well, it should be clear that passive components should be able to withstand the high amounts of currents and voltages during switching transitions.
In these inverters, the current ripple across the output inductor creates significant amount of power losses. The impact of this current ripple on the power losses is two-fold. First, the current ripple produces core losses and, second, the current ripple also increases the high frequency ohmic losses of the converter. Usually, magnetic wires are used to wind the output inductors. The magnetic wires show a significant resistance to the high frequency current. In conventional designs, the current ripple is usually kept very small so that only a negligible high frequency current ripple is produced. As well, in conventional designs, the current ripple is usually kept very small so that only a small amount of the attributed high frequency ohmic losses are produced Unfortunately, another restriction is that, in high power density designs, the inductor cannot be very bulky.
What is needed is a simple and practical solution which provides soft-switching for the power semiconductors and simultaneously does not compromise system reliability. Also, the high frequency current component of the output inductor should be limited in order to provide a high power density converter with superior efficiency. It is therefore preferable to only use passive components for soft-switching the power semiconductors and to also reduce the high frequency component of the inverter output current. Also, another preference is to optimize (or minimize) any losses associated with the extra passive components. Such preferred optimized losses due to the extra components should not offset the advantages of soft-switching.